Tuesday, February 24, 2009

Aero Road

I really enjoy Lennard Zinn's articles on VeloNews. I don't always agree with his views, but I often find insight there. Today was an exception.

Kestrel's latest Talon SL: very slick.

He was asked about the benefit of aerodynamic road frames for mass-start racing. These frames claim to save approximately 1% to 2% of total wind resistance. There's a bunch of them. Kestrel was really the first to my knowledge with their carbon fiber Talon, although that failed to catch on among the mass-start crowd. Cervelo was the first to really break through with its aluminum Soloist, followed by Soloist Carbon (SLC), SLC-SL, and finally this year the S-series. More recently, Felt (with its AR) and Ridley (with its Noah) have jumped on board. There's still others. Chris Boardman claims to be testing an aero road frame.

Yet the pros are sometimes reluctant to take advantage of these. While Cervelo was largely successful with its SLC, and even more so with its S3, at getting pro riders on their aero frame, on Garmin, for example, despite the much-publicized AR series debut at last year's Tour, this year (so far) the riders are all on the "conventional" F1. And when Kestrel broke into the pro ranks with its Rock Racing sponsorship, it's with the RT-900 as opposed to the Talon or Talon SL.

So why? Lennard says aero isn't a benefit in any significant cross-wind. He cites data on the Hed web site, that resistance of aero rims begins to increase above 8 degrees yaw. Let's check out some of the data he references, using Zipp wheels to limit the number of variables other than rim depth:

As Zinn notes, the resistance does increase for the deepest dish rims above a certain yaw, in this case 10 degrees. However, an advantage over shallower dish is retained out to at least 20 degrees. For a racing bike moving along at typically 10 meters per second or more, a 20 degree yaw represents a near-surface cross-wind component of 3.6 meters per second. That's 3.3 meters per second even if longitudinal relative wind is reduced 10%, a ballpark number for drafting benefit. Basically, 20 degree yaw relatively large under the vast majority of conditions.

The pros seem to agree. For essentially all mass-start races from tight criteriums to epic Tour stages, riders are on deep dish wheels. The days of 25.4 mm are long gone. The only evident exception is the cobbled classics like Paris-Roubaix and Flanders, where while deep carbon is becoming popular, old school metal rims are still relatively common due to their reliability.

Zinn argues that the mass penalty of profiled tube road frames makes them an unwise choice for criteriums, where accelerating out of corners makes low mass a premium. Lennard, Lennard, Lennard. I know you know how to run numbers.

Consider sprinting out of a tight corner from 30 kph to 45 kph over 5 seconds. For a rider + bike totaling even 75 kg, that's 650 watts into acceleration alone, not counting wind resistance. But the mass difference of an aero frame and post relative to a non-aero frame and post is only around 150 grams (compare the Kestrel Talon SL to the Fuji SL-1, for example, each relatively unpainted carbon road frames: paint would increase the differential). The extra power to accelerate the heavier frame over the same 5 seconds is only 1.3 watts. Yet wind tunnel data at multiple yaw angles shows the aerodynamic frame saves at least this much at these speeds. So even in the tight corner the aero frame may be better. The rest of the lap, the aero benefit is just icing on the cake.

So why aren't the pro riders jumping all over the aero revolution? The obvious answers are comfort, stiffness, handling, and/or geometry. Pros like stiff frames, generally, perhaps not for simple reasons of energy transfer efficiency (analysis has shown that substantial majority of energy going into frame flex is returned through the drivetrain into propulsion), but in how the bike feels. With Cervelo, the geometry is the same between the S (aero) and R (conventional) series, but for example with Felt the two differ substantially, the AR having longer head tubes at the same front-center distance. That may be an issue with Garmin.

So I think Zinn got lazy here. He started with the answer he wanted, then kludged reasoning to justify that. But the question is obviously more complicated. From wind tunnel data, you'd think every cat 4 would be on an aero frame just to be competitive. But at the top, pros keep winning races on "conventional" frames.